Rice and Salinity in the Sahel

Rice lands in the Sahel are once again being threatened. This
time, however, it is not drought or pests but a more preventable
problem, salinization. Since 1990, the results of the structural
adjustments recommended by the International Monetary Fund and
the World Bank are becoming evident. These agencies have urged
the Sahelian countries of Senegal, Mauritania, Niger, Mali and
Burkina Faso to transfer rice production from public to private
hands. In the salinity-prone Senegal River delta, privatization
of the rice sector has resulted in the initiation of cheap irrigation
schemes which frequently are so poorly constructed that they sometimes
last for only one season. Poor water management, the absence of
efficient drainage and the high cost of irrigation have led to
rising groundwater tables and soil sodification and alkalinization,
leaving areas unsuitable for future cultivation.

Thousands of hectares in the Sahelian floodplains are being used
to grow rice. The enormous demand for the crop has led to a doubling
of the region's output from 352,000 tons of rice (1980) to approximately
800,000 tons (1992), replacing millet and sorghum as the staple
crop in Sahelian cities and some rural areas. About 150,000 hectares
of the Sahel are under rice cultivation and an equal area is cultivated
with partial water control. Although this represents only 5% of
the area under rice cultivation in West Africa, the Sahelian fields
yield about 12% of the region's total rice output. Continued increases
in rice production were anticipated with the implementation of
structural adjustments. However, it now seems that this increase
in rice production is unsustainable, and that current cultivation
methods could damage the land's production capabilities.

Rising Water

The annual rainfall in the Sahel is between 150 and 600 mm --
insufficient for the cultivation of upland or rainfed-lowland
rice. The clay floodplains associated with the major, permanent
rivers in the Sahel, however, are suited to grow rice under flooded
conditions. A canal irrigation system is used, typically with
an elevated principal tributary. Water is pumped from the rivers
or diverted from dams.

Less than one-fourth of the more fertile and irrigable clay soils
in the Sahel have been developed for irrigation. But wherever
water and markets are easily accessible, pressure on the land
is high. Correspondingly, farmers grow rice even on sandy soils
which are not suited for irrigation. Uncontrolled irrigation of
plots with highly permeable soils, particularly since the liberalization
of production, and the growing practice of keeping principal canals
flooded year-round, have resulted in a dramatic rise in the groundwater
table.

In the 1940s, groundwater tables in the largest irrigation schemes
in Mali were between 30 and 50 meters deep. They oscillated with
the seasons but always returned to their original level. Today,
groundwater tables are around 1 meter below the surface at the
center of irrigation systems with little to no oscillation. Even
20 kilometers away from the irrigated lands, the water table has
been rising at a rate of 0.5 meters annually.

The agronomic consequences of the rising water table are grave
for two reasons. First, the groundwater is high in dissolved salts
due to leachate, and salinizes the topsoil by capillary rise.
This is generally irreversible at sites where topography does
not permit lateral drainage. Secondly, alternative crops such
as vegetables, maize, sugar cane and cotton can no longer be grown
due to a lack of oxygen in the soil. On lands under the Niger
Office in Mali, people are largely reduced to planting a rice
monoculture.

Saline and Alkaline Soils

In Mauritania and the Sahelian parts of Senegal, the rice fields
lie on the banks of the Senegal river. Most of the cultivated
surfaces were prone to seasonal transgressions of saltwater from
the sea until completion of two dam projects in the late 1980s,
the Diama and the Manantali dams. River regulation reduced farmers'
opportunities for traditional flood-recession farming, but enabled
the growing of irrigated crops throughout the year.

For private rice producers who took advantage of market liberalization,
cost of irrigation is roughly proportional to water consumption.
Because they operate their own pumps, they hesitate to remove
the valuable water from their plots, leaving it to evaporate and
saline concentrations to accumulate.

Even highly salinized soils can be fully regenerated with appropriate
water management. But a very disturbing development was observed
on the irrigated areas in Mali. Previously acid clay soils turned
alkaline, almost entirely losing their organic matter content
and physical structure through solubilization. Sodification/alkalinization,
a type of salinization, is associated with nearly irreversible
soil degradation. It is driven by the latent alkalinity and sodium
adsorption ratio (SAR) of irrigation water which, over years or
decades, replaces the exchangeable bases in the minerals with
sodium and precipitates insoluble carbonate.

Water table samples from Mali were found to have SAR values of
10 to 50, and a pH level ranging from just below 8.5 to 10. Over
the past 30 years, the porosity of samples collected at Mali's
Niger Office has declined by 30%, while the permeability has been
halved. The soils have formed a compact concrete-like crust which
makes plowing, planting and root penetration difficult. The high
pH, due to sodification/alkalinization, also reduces plant nutrient
availability and increased nitrogen losses through volatilization.

Both sodification/alkalinization and rising groundwater tables
were not anticipated when the huge irrigation schemes in Mali
were designed without provision for drainage. The groundwater
level seemed too deep and the irrigation water too pure to pose
any such risks. It is now clear that the waters of the Niger and
Senegal rivers carry substantial alkalinity, and their salt content
sometimes increases markedly between the main rivers and the actual
irrigation site. Alkalinization and sodification are more advanced
at the Niger Office than elsewhere in the Sahel because these
are the oldest irrigation systems in the area. The same development,
however, is likely to occur in the middle and upper valley of
the Senegal River if no preventive measures are taken.

Solutions

Although the causes of soil degradation in Sahelian rice irrigation
systems are diverse, preventive and regenerative solutions generally
hinge on improved water and irrigation management. Soil regeneration
is economically possible where salinity is not associated with
severe alkalinity, and where a saline groundwater table is not
interactive with the topsoil. But some highly degraded soils in
the Niger Office's areas in Mali, sometimes with a pH of 9 or
more, can only be regenerated with large amounts of organic matter
and gypsum or the production of halophytes such as amshot, Echinochloa
stagninum (Ag-Sieve Vol. 5 No. 5).

Systematic drainage-irrigation cycles during a cropping season
and evacuation of water from irrigation canals during the off
season need to be encouraged.

Farmers need to be educated on the risks of poor water and soil
management. More research is needed on decision-making processes
and land tenure rights in rice growing communities before technologies
will be adopted that bring about long-term economic returns.